1. Technical Field
The present invention relates to a method for controlling a generator of electric energy that is connected to an electrical supply grid on a grid connection point. Furthermore, the present invention relates to a generator of electric energy that is connected to an electrical supply grid.
2. Description of the Related Art
Feeding electric energy into an electrical supply grid, such as the European grid or the US power grid, is generally known. The electrical supply grid as described below refers to the widely accepted AC voltage grid. This does not exclude the presence of DC voltage sections in the grid. Likewise, frequency-independent aspects may generally also refer to a DC voltage grid. Historically, energy is fed into an electrical supply grid with a large power station that drives a synchronous generator using primary energy, such as coal, nuclear energy or gas. Depending on the number of pole pairs and the speed of the synchronous generator, this generator feeds into the supply grid with a certain frequency. The synchronous generator may be technically controlled so as, for example, to adjust the output. However, this adjustment process can take a long time.
With changing situations in the supply grid, the physical reaction of the synchronous generator often causes a change in the grid condition, at least for a short time. For example, the speed of the synchronous generator increases if the supply grid is not able to take the power completely that is or can be provided by the synchronous generator. This excess power then accelerates the synchronous generator, which results in an increased feeding frequency. Accordingly, the frequency in the supply grid may increase.
When feeding into a supply grid, the network stability typically must also be taken into account. The loss of network stability, i.e., the loss of the stability of the supply grid, may result in the feeding generator being powered off. Such a loss of stability, which is referred to as such and abbreviated as “LOS” among German-speaking experts, describes physical processes that no longer allow for a continued operation and must be terminated by cutoffs. In the case of power plants, this affects their output, and can thus contribute to an escalation of the so-called deficit output. In the worst case, this loss of stability leads to a total energy system failure due to error cascading and deficit accumulation. Such total failures are very rare; however, one occurred in Italy on 24 Sep. 2004.
Loss of network stability, i.e., the so-called loss of stability, is a phenomenon which involves a loss of angular stability that may eventually cause a loss of voltage stability.
Overcurrents to be achieved are determined as stability criteria, which must be provided in the case of a loss of stability. This requires the system to have a certain design. A new power plant, in particular a power plant that is to be newly built, is thus coordinated to the supply grid as is represented on the grid connection point to which the power plant is to be connected.
When connecting large power plants to an electrical supply grid, the short circuit current ratio is an important criterion; this is known among German specialists as “short circuit radio” and abbreviated as “Scr”. This short circuit current ratio is the ratio of the short circuit power to the connected load. Short circuit power is the power that the respective supply grid on the considered grid connection point, to which the power plant is to be connected, can provide in the case of a short circuit. The connected load is the connected load of the power plant that is to be connected, in particular the nominal capacity of the generator that is to be connected.
To secure reliable operation, i.e., to avoid a loss of stability to the greatest extent possible, power plants are generally designed for the respective grid connection point in such a way that the short circuit current ratio is higher than 10, normally even higher than 15. The supply grid can then provide a relatively high short circuit power on the grid connection point. That means that the grid has a low impedance and is referred to as a strong grid.
In the case of a weak grid (in other words, in the presence of a high impedance), feeding is only possible with a low connected load, i.e., only a power plant with a low connected load can be connected. This usually leads to the fact that either a new power plant is not connected to such a grid connection point, or the grid has to be changed, particularly by equipping it with further, more powerful lines. This is generally referred to as grid reinforcement.
For feeding electric energy by decentralized production units, in particular wind power installations, the problem of the grid's loss of stability is basically unknown. Already at the end of the nineties, first proposals were made to ensure that wind power installations also contribute to the electrical support of the grid. This, however, does not take into account the cause of a loss of stability, in particular that feeding into the supply grid can cause a loss of stability.
For example, the German patent application U.S. Pat. No. 6,891,281 describes a method in which wind power installations can change and, in particular, reduce their power feed-in. U.S. Pat. No. 7,462,946 suggests that in the case of a grid failure, particular in the case of a short circuit, a wind power installation limits the power that it feeds-in instead of being disconnected from the grid in order to achieve a grid support. U.S. Pat. No. 6,965,174 describes a method for supporting the grid by means of a wind power installation that, depending on the grid voltage, adjusts a phase angle of the fed-in electricity, and thus feeds reactive power into the grid depending on the voltage so as to support the grid. U.S. Pat. No. 6,984,898 also relates to a method for supporting the grid by means of a wind power installation in which the wind power installation reduces, depending on the grid voltage, the power that is to be fed into the grid, particularly so as to avoid a disconnection from the grid in order to support the grid by means of a wind power installation.
The fact that such decentralized production units, such as wind power installations, may be the underlying cause for the loss of stability in the grid has not been taken into account. In the essay “Loss of (Angle) Stability of Wind Power Installations” by V. Diedrichs et al., submitted for and presented at the “10th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission Networks for Offshore Wind Farms, Aarhus (Denmark), 25-26 Oct. 2011”. There, reference was basically made to the problem that the loss of stability in the grid can basically also occur for wind power installations that are connected to the supply grid for feed-in. This essay essentially raises awareness of the problem and is hereby incorporated herein by reference in its entirety. In particular, its technical explanations also apply to the present application.
Basically, findings, experience and other knowledge of the operation and connection of large power plants to the electrical supply grid cannot be transferred to wind power installations, including large wind parks with numerous wind power installations that are connected to the supply grid for feed-in. The responsible expert who connects a power plant to a supply grid in order to operate it there is already a different expert from the one wanting to connect a wind power installation to the supply grid in order to operate it there. Wind power installations—and much of the following also applies to decentralized production units—depend on wind and therefore take a fluctuating energy source into account; they usually do not feed into the supply grid with a synchronous generator that is directly coupled to the grid, but use a voltage-based inverter instead; they have a different size than large power plants, whereby their nominal capacity is usually 3 powers of ten below that of a large power plant; they are usually subject to other political laws which often ensure the acceptance of the provision of service by the operators of electrical supply grids; they are usually decentralized; and, they usually feed into a medium-voltage grid, whereas large power plants usually feed into an extra high voltage grid.
If a loss of stability that is to be avoided occurs in spite of all due caution and precautionary measures, this leads to a problematic situation. If such a loss of stability occurs, the generator of electric energy must be switched off for the concerned grid connection point. Such a switching off is triggered according to predetermined criteria, and the respectively concerned generator monitors said criteria, and switches off if it has recognized that said criteria are met. However, the power that is immediately fed into the grid, and thus the existing power in the grid, changes. The loss of said power of said generator can lead to the fact that further points of common coupling nearby meet the criteria for switching off, and accordingly switch off further generators, which can result in even further generators being switched off, which can finally result in a complete blackout of the entire supply grid.
The German Patent and Trademark Office has found the following prior art in the priority application for the present application: DE 10 2009 027 981 B4, DE 10 2008 062 356 A1, WO 2011/050807 A2 and DE 10 2008 045 938 A1.